Piston type pressure increasing or decreasing units are generally constituted by at least one master cylinder, in which there can move a master piston which is rigidly connected to at least one slave piston which can move in a slave cylinder, the two pistons moving over the same path but having a different cross-section. Each piston cooperates with a cylinder and a cylinder head in order to form a closed and sealed space having a variable volume. The master cylinder communicates with a hydraulic circuit which is independent of the circuit of the slave cylinder.
Piston type pressure increasing or decreasing units may be used in a static manner in order to maintain two circuits or two volumes which are independent of each other at a constant pressure ratio without there necessarily being established a flow of hydraulic fluid which involves the displacement of the master piston and the slave piston.
In the case of piston type pressure increasing units which convert a hydraulic fluid flow into a hydraulic fluid flow which is smaller but under greater pressure, or in the case of the or piston type pressure decreasing units which convert a hydraulic fluid flow into a hydraulic fluid flow which is greater but under lower pressure, the master piston constitutes a hydraulic motor which converts a hydraulic fluid flow into movement, the movement being communicated to the slave piston which forms a hydraulic pump so as to convert the movement into a hydraulic fluid flow. In order to increase the pressure, the master piston must have a cross-section larger than that of the slave piston whilst, in order to reduce the pressure, the master piston must have a cross-section smaller than that of the slave piston.
In this case, it may be noted that the master cylinder comprises at least one inlet and at least one outlet which may each be kept open or closed by a valve while the slave cylinder has at least one inlet which comprises a non-return valve which allows the hydraulic fluid to be introduced into the cylinder but not to be discharged therefrom and at least one outlet which comprises a non-return valve which allows the hydraulic fluid to be discharged from the cylinder but not to be introduced therein.
When a flow is durably established through the piston type increasing or decreasing units, the operation thereof is sequential because, when the pistons which they comprise reach the travel end, the pistons must return to the travel start, and vice versa, as long as the increasing or decreasing units operate. That sequential operation is responsible for losses of energy which are undesirable owing to the compressibility of the hydraulic fluid, the losses being proportionally greater when the fluid is compressible and the pressures used are high. For the same operating pressures, the losses are proportionally greater if a pressure decreasing unit is involved, the losses occurring mainly in the region of the master cylinder of the decreasing unit.
In practice, for pressures of several tens or hundreds of bar, the performance of the piston type pressure increasing or decreasing units remains high. When those piston type pressure increasing or decreasing units are used at pressures which are still higher—for example, in the order of a thousand bar or more—the compression rate of the hydraulic fluid is increased, which further worsens the performance, even when fluids which are known to be poorly compressible, such as oil or water, are used.
This is because energy is stored during the compression of the hydraulic fluid, but the energy is normally lost at the end of travel of the pistons, mainly at the side of the master piston. That results from the fact that, when the piston arrives at the end of travel, the master cylinder in which it moves is completely filled with pressurized fluid. Therefore, so that the master piston can move off again in the opposite direction, it is first necessary to decompress the fluid contained in the cylinder. The loss of energy results from the inability to convert the compression energy of the fluid into an additional flow of pressurized fluid which is available at the outlet of the slave cylinder, unless the whole of the circuit which is connected to the outlet of the slave cylinder is decompressed at the same proportions, which is rarely possible.
This is because, in practice, when the master piston arrives at the end of travel, the master cylinder thereof is decompressed in a low-pressure circuit without compensation during operating production, and the compression energy stored in the hydraulic fluid is dissipated in the form of heat. In accordance with the application considered, that loss makes the use of pressure increasing or decreasing units largely irrelevant.
In this regard, it would be particularly advantageous to be able to recover this compression energy, particularly involving piston type pressure increasing or decreasing units which operate under very high pressures.
For example, the reversible hydraulic pressure converter having tubular valves, to which the patent application No. 1358071 of 20 Aug. 2013 belonging to the same Applicant relates, would have its energy performance level substantially increased if it cooperated with recovery means for the compression energy of the hydraulic fluid, whatever the context of the application of the converter. It should further be noted that, if the converter is used in order to produce motor vehicles with hydraulic hybrid transmission with storage/restitution of pressurized oil, it becomes particularly advantageous to recover the compression energy of the hydraulic fluid in the converter, which allows the consumption of fuel per kilometer of the vehicles to be reduced.
The advantage in terms of energy brought about by recovery means for the compression energy of the hydraulic fluid would also benefit any sequential pressure converter, increasing unit or decreasing unit with pistons, whatever the number of master pistons or slave pistons which it comprises, and whatever the field of application thereof.